In-situ containment

In situ grouting. The in situ grouting method involves injecting grouting materials into sediments to stabilize the contaminated sediments. In situ containments can be either temporary or permanent. However, permanent containment of contaminated sediments has not been well demonstrated or widely used. 

Since the early preparation of TPPMS by Chatt and co-workers [1] in 1958 and the spectacular properties of TPPTS discovered by Kuntz in 1975 [2, 3], many complexes have been prepared, most of them in situ, containing in addition to the water-soluble phosphines other ligands required for catalysis, such as hydride, carbon monoxide, alkene, etc. 

Biological functions of the structure. The trematode tegument is structurally adapted for transport, immune evasion and communication with the neuromuscular system (via gap junctions). Numerous pits at the surface of the tegument markedly increase the surface area of the parasite, which is consistent with a transport function (see below). The fact that all of the pits in schistosomes examined in situ contain erythrocytes (55) suggests that these structures are open to the external environment. The tegument is less pitted in F. hepatica, but numerous invaginations of the surface effectively increase the surface area. 

In-situ containment, in which sediment contaminants are in some manner isolated from target organisms, though the sediments are left in place. 

In Situ Containment and Stabilization Conceptual Approach Net Reaction... 

Figure 1. Conceptual model for in situ containment and stabilization of divalent cations in caldte minerals using microbial-based urea hydrolysis.

Ammonia—Gas-Cured Flame Retardants. The first flame-retardant process based on curing with ammonia gas, ie, THPC—amide—NH, consisted of padding cotton with a solution containing THPC, TMM, and urea. The fabric was dried and then cured with either gaseous ammonia or ammonium hydroxide (96). There was Httle or no reaction with cellulose. A very stable polymer was deposited in situ in the cellulose matrix. Because the fire-retardant finish did not actually react with the cellulose matrix, there was generally Httle loss in fabric strength. However, the finish was very effective and quite durable to laundering. 

Decabromodiphenyl Oxide—Polyacrylate Finishes. An alternative to the diffusion technique is the appHcation of decabromodiphenyl oxide on the surface of fabrics in conjunction with binders (131). Experimental finishes using graft polymerization, in situ polymerization of phosphoms-containing vinyl monomers, or surface halogenation of the fibers also have been reported (129,130,132,133). 

Figure 4d represents in situ encapsulation processes (17,18), an example of which is presented in more detail in Figure 6 (18). The first step is to disperse a water-immiscible Hquid or soHd core material in an aqueous phase that contains urea, melamine, water-soluble urea—formaldehyde condensate, or water-soluble urea—melamine condensate. In many cases, the aqueous phase also contains a system modifier that enhances deposition of the aminoplast capsule sheU (18). This is an anionic polymer or copolymer (Fig. 6). SheU formation occurs once formaldehyde is added and the aqueous phase acidified, eg, pH 2—4.5. The system is heated for several hours at 40—60°C.

Domestic. Estimates of U.S. uranium resources for reasonably assured resources, estimated additional resources, and speculative resources at costs of 80, 130, and 260/kg of uranium are given in Table 1 (18). These estimates include only conventional uranium resources, which principally include sandstone deposits of the Colorado Plateaus, the Wyoming basins, and the Gulf Coastal Plain of Texas. Marine phosphorite deposits in central Elorida, the western United States, and other areas contain low grade uranium having 30—150 ppm U that can be recovered as a by-product from wet-process phosphoric acid. Because of relatively low uranium prices, on the order of 20.67/kg U (19), in situ leach and by-product plants accounted for 76% of total uranium production in 1992 (20). 

The in situ process is simpler because it requires less material handling (35) however, this process has been used only for resole resins. When phenol is used, the reaction system is initially one-phase alkylated phenols and bisphenol A present special problems. As the reaction with formaldehyde progresses at 80—100°C, the resin becomes water-insoluble and phase separation takes place. Catalysts such as hexa produce an early phase separation, whereas NaOH-based resins retain water solubiUty to a higher molecular weight. If the reaction medium contains a protective coUoid at phase separation, a resin-in-water dispersion forms. Alternatively, the protective coUoid can be added later in the reaction sequence, in which case the reaction mass may temporarily be a water-in-resin dispersion. The protective coUoid serves to assist particle formation and stabUizes the final particles against coalescence. Some examples of protective coUoids are poly(vinyl alcohol), gum arabic, and hydroxyethjlceUulose. 

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